Auto merge of rust-lang#103556 - clubby789:specialize-option-partial-eq, r=scottmcm

Manually implement PartialEq for Option<T> and specialize non-nullable types

This PR manually implements `PartialEq` and `StructuralPartialEq` for `Option`, which seems to produce slightly better codegen than the automatically derived implementation.

It also allows specializing on the `core::num::NonZero*` and `core::ptr::NonNull` types, taking advantage of the niche optimization by transmuting the `Option<T>` to `T` to be compared directly, which can be done in just two instructions.

A comparison of the original, new and specialized code generation is available [here](https://godbolt.org/z/dE4jxdYsa).

Author: bors

core/src/option.rs

@@ -512,7 +512,7 @@ use crate::{
 };
 
 /// The `Option` type. See [the module level documentation](self) for more.
-#[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
+#[derive(Copy, PartialOrd, Eq, Ord, Debug, Hash)]
 #[rustc_diagnostic_item = "Option"]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub enum Option<T> {
@@ -2035,6 +2035,72 @@ impl<'a, T> const From<&'a mut Option<T>> for Option<&'a mut T> {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T> crate::marker::StructuralPartialEq for Option<T> {}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: PartialEq> PartialEq for Option<T> {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        SpecOptionPartialEq::eq(self, other)
+    }
+}
+
+#[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")]
+#[doc(hidden)]
+pub trait SpecOptionPartialEq: Sized {
+    fn eq(l: &Option<Self>, other: &Option<Self>) -> bool;
+}
+
+#[unstable(feature = "spec_option_partial_eq", issue = "none", reason = "exposed only for rustc")]
+impl<T: PartialEq> SpecOptionPartialEq for T {
+    #[inline]
+    default fn eq(l: &Option<T>, r: &Option<T>) -> bool {
+        match (l, r) {
+            (Some(l), Some(r)) => *l == *r,
+            (None, None) => true,
+            _ => false,
+        }
+    }
+}
+
+macro_rules! non_zero_option {
+    ( $( #[$stability: meta] $NZ:ty; )+ ) => {
+        $(
+            #[$stability]
+            impl SpecOptionPartialEq for $NZ {
+                #[inline]
+                fn eq(l: &Option<Self>, r: &Option<Self>) -> bool {
+                    l.map(Self::get).unwrap_or(0) == r.map(Self::get).unwrap_or(0)
+                }
+            }
+        )+
+    };
+}
+
+non_zero_option! {
+    #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU8;
+    #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU16;
+    #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU32;
+    #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU64;
+    #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroU128;
+    #[stable(feature = "nonzero", since = "1.28.0")] crate::num::NonZeroUsize;
+    #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI8;
+    #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI16;
+    #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI32;
+    #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI64;
+    #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroI128;
+    #[stable(feature = "signed_nonzero", since = "1.34.0")] crate::num::NonZeroIsize;
+}
+
+#[stable(feature = "nonnull", since = "1.25.0")]
+impl<T> SpecOptionPartialEq for crate::ptr::NonNull<T> {
+    #[inline]
+    fn eq(l: &Option<Self>, r: &Option<Self>) -> bool {
+        l.map(Self::as_ptr).unwrap_or_else(|| crate::ptr::null_mut())
+            == r.map(Self::as_ptr).unwrap_or_else(|| crate::ptr::null_mut())
+    }
+}
+
 /////////////////////////////////////////////////////////////////////////////
 // The Option Iterators
 /////////////////////////////////////////////////////////////////////////////